There are motors of this kind, which are optimized for operation in one rotational direction ("preferred direction") and therefore starting up in the opposite direction, referred to below as the "reverse direction", is either impossible or can only be achieved with difficulty, so that starting in the reverse direction is not reliably assured. These motors operate with an auxiliary torque which has a very unfavorable course when operating in the reverse direction. This auxiliary torque is usually formed as a reluctance auxiliary torque. However, a mechanically generated auxiliary torque would also be possible within the scope of the invention.
Endeavors have been made to give motors of this kind the capacity to start in both rotation directions by virtue of an auxiliary pole being provided between their main poles, see DE 32 35 774 C2, Muller/Papst Motoren, but motors of this kind are costly due to their special construction and for this reason, have not been able to gain acceptance in actual use.